Digital subscriber line (DSL) technologies can provide relatively large bandwidth for digital communications over existing subscriber lines. When transmitting data over the subscriber lines, crosstalk interference can occur between the transmitted signals over adjacent twisted-pair phone lines, for example in a same or nearby bundle of lines. Noise and crosstalk in the lines can limit the performance of DSL systems. The noise and crosstalk can be highly non-stationary and vary dramatically, which causes instability in the DSL systems. During active line communications, also referred to as showtime, bit swapping (BS) and/or seamless rate adaption (SRA) may be used to stabilize the system by adjusting bit-loading in a dynamic manner. However, in the case of rapidly changing noise conditions in the lines, BS and SRA may not be fast enough to adapt to the changing line conditions. Therefore, it may be necessary to re-initialize the lines in such conditions, e.g. by deactivating and reactivating the lines, which causes undesirable interruptions in services.
One method to ensure line stability and reduce line re-initialization is to reserve extra noise margin during line initialization and showtime. However, this method can substantially reduce the achievable data rates in the lines. Another method is to use virtual noise (VN), a technique that limits the maximum bit-loading allowed on each tone in the line, e.g. based on the International Telecommunication Union (ITU) Telecommunication Standardization Sector (ITU-T) G.992.3 and G.992.5 standards, which are incorporated herein by reference. The VN method is used to improve DSL stability by ensuring operation of a line at a bit-loading level that can be sustained when worse noise conditions are encountered. As such, different VN levels can be determined for different frequency ranges, e.g. where higher VN levels are applied for frequencies with higher noise. The VN method may allow higher data rates than the extra noise margin method since the VN method can be applied selectively to different bins or tones. However, the VN method requires a communication channel between a central office (CO) transmitter and a customer premise equipments (CPE) receiver, which should be configured to support the VN function. In some legacy systems, the VN method may not be used since the CPEs do not support the VN function.